Belgian federal magistrate Eric Van der Sypt said in Brussels that there was no link at this stage between the Paris attacks and the Belgian operation, which he said is the result of an investigation that has been underway for a few weeks.
The suspects immediately opened fire on police when they closed in on them near the city's train station, he said. There was an intense firefight for several minutes on an upper level of a building in Verviers where the raid took place, which appears to be residential.
"These were extremely well-armed men" with automatic weapons, Van der Sypt said. Police buildings were the target of an attack expected within hours or days, he said.
"We still expect a number of arrests," he said. No police were wounded or killed in the firefight, which occurred at the height of rush hour in a crowded neighborhood. Verviers, a former industrial town with about 56,000 residents including a large immigrant community, is about 125 kilometers (80 miles) southeast of the capital,Brussels.
The magistrate said more anti-terrorist raids were underway in the Brussels region, adding that Belgium's terror alert level was raised to its second-highest level. The operation was part of an investigation into extremists returning from Syria, authorities said.
Witnesses speaking on Belgium's RTBF radio described a series of explosions followed by rapid fire at the center of Verviers, near a bakery and in the neighborhood of the train station. Video posted online of what appeared to be the raid showed a dark view of a building amid blasts, gunshots and sirens, and a fire with smoke billowing up.
Earlier Thursday, Belgian authorities said they are looking into possible links between a man they arrested in the southern city of Charleroi for illegal trade in weapons and Amedy Coulibaly, who prosecutors say killed four people in a Paris kosher market last week.
The man arrested in Belgium "claims that he wanted to buy a car from the wife of Coulibaly," Van der Sypt said. "At this moment this is the only link between what happened in Paris." Van der Sypt said that "of course, naturally" we are continuing the investigation.
At first, the man came to police himself claiming there had been contact with Coulibaly's common law wife regarding the car, but he was arrested following a search on his premises when enough indications of illegal weapons trade were found.
A Belgian connection figured in a 2010 French criminal investigation into a foiled terrorist plot in which Coulibaly was one of the convicted co-conspirators. The plotters included a Brussels area contact who was supposed to furnish both weapons and ammunition, according to French judicial documents obtained by The Associated Press.
Several countries are now involved in the hunt for possible accomplices to Coulibaly and the two other gunmen in the French attacks.
In Spain, authorities said Coulibaly drove his common-law wife from France to Madrid on Dec. 31 and was with her until she took a Jan. 2 flight to Istanbul.
Spain's National Court said in a statement it was investigating what Coulibaly did in the country's capital with his wife, Hayat Boumeddiene, and a third person who wasn't identified but is suspected of helping Boumeddiene get from Turkey to Syria.
France is on edge since last week's attacks, which began Jan. 7 at the satirical newspaper Charlie Hebdo. The paper, repeatedly threatened for its caricatures of the Muslim Prophet Muhammad, buried several of its slain staff members Thursday even as it reprinted another weekly issue with Muhammad on its cover.
Defense officials said Thursday that France was under an unprecedented cyber assault with 19,000 cyberattacks launched after the country's bloodiest terrorist attacks in decades, frustrating authorities as they try to thwart repeat violence.
Around 120,000 security forces are deployed to prevent future attacks.
Calling it an unprecedented surge, Adm. Arnaud Coustilliere, head of cyberdefense for the French military, said about 19,000 French websites had faced cyberattacks in recent days, some carried out by well-known Islamic hacker groups.
The attacks, mostly relatively minor denial-of-service attacks, hit sites as varied as military regiments to pizza shops but none appeared to have caused serious damage, he said. Military authorities launched round-the-clock surveillance to protect the government sites still coming under attack.
Two of the Paris terror attackers claimed allegiances to al-Qaida in Yemen and a third to the Islamic State group.
The terror attacks in Paris occurred in an atmosphere of rising anti-Semitism in France and have prompted scattered retaliatory violence against Muslims, and Muslim sites around France. Justice officials have also been cracking down by arresting dozens of people who allegedly glorified terrorism, or made racist or anti-Semitic remarks.
French President Francois Hollande insisted Thursday that any anti-Muslim or anti-Semitic acts must be "severely punished." He said France's millions of Muslims should be protected and respected, "just as they themselves should respect the nation" and its strictly secular values.
Raf Casert reported from Brussels. John-Thor Dahlburg, Greg Keller, Jamey Keaten, Angela Charlton, Sylvie Corbet, Lori Hinnant, Matthew Lee and Nicolas Vaux-Montagny in Paris, Nicole Winfield on the papal plane, Jorge Sainz in Madrid, contributed to this report.
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